Field of the Invention
The invention relates to a cryostat having a magnet arrangement for the generation of a magnetic field B0, wherein the magnet arrangement comprises a low-temperature superconductor (LTS) portion having at least one LTS section made from a conventional low-temperature superconductor and a high-temperature superconductor (HTS) portion having at least one HTS section made from a high-temperature superconductor, wherein the HTS portion is arranged radially within the LTS portion, and wherein the cryostat is designed to control the temperature of the LTS portion and the HTS portion independently of one another. Such a cryostat has been disclosed in DE 10 2006 012 508 B3.
Description of the Related Art
Nuclear magnetic resonance (=NMR) spectroscopy is a powerful method of instrumental analysis which is based on the alignment of nuclear spins in a strong magnetic field and their behavior when radiated with high-frequency pulses. NMR magnets based on superconducting coils are normally used to generate particularly high magnetic field strengths which are required for high spectral resolutions.
Conventional NMR magnets are normally produced from low-temperature superconductor (=LTS) wires, in particular NbTi or Nb3Sn wires. As a result of the critical field of Nb3Sn, there is currently an upper limit of approximately 23.5 T for the achievable magnetic field strength which, in NMR, corresponds to a proton resonance frequency of 1000 MHz.
In order to achieve higher field strengths or to enable a magnet with a given field strength to be made more compact, alternative conductor materials must be called upon. In this context, research is mainly carried out into the use of high-temperature superconductor (=HTS) tape conductors, in particular based on YBCO. Coils in the form of a solenoid are normally wound with the HTS tape conductors.
In doing so, the NMR magnet is not made entirely from HTS materials; for cost reasons, it is advantageous to draw upon HTS only for the innermost sections and produce the background magnet in conventional LTS technology based on NbTi or Nb3Sn. Normally, a magnet section is wound from HTS tape material and then connected electrically in series with an LTS background magnet. For example, cryostats having magnetic coil systems in which, in each case, at least one inner HTS section is connected in series with at least one outer LTS section, are described in DE 10 2006 012 508 B3, DE 10 2006 012 509 B3 and in DE 10 2006 012 511 B3.
HTS tape conductors can only be produced in limited lengths so that, as a rule, superconducting or at least very low resistance connecting points (joints) of HTS to HTS are required for a typical NMR magnetic coil, which is technologically difficult. Likewise, in the known magnetic coil systems, superconducting or at least very low resistance connecting points (joints) from LTS to HTS must be produced, which is likewise technologically difficult. In general, a location with low magnetic field strength must be chosen for the joints, which often makes complex conductor routing necessary. Furthermore, as a rule, tape conductors have varying properties over the cross section, which leads to inhomogeneities in the generated magnetic field. Likewise, shielding currents in the tape conductors can lead to inhomogeneities over the width of the tape.
HTS coils which are not based on wound tape conductors are also known from the prior art.
DE 100 33 869 A1 describes a system of HTS rings which are charged by means of current pulses via connecting pieces of the rings which are positioned closely together in the circumferential direction or by magnetic pulses from a normal conducting copper coil.
U.S. Pat. No. 6,489,769 B2 describes an HTS magnet having a hollow, cylindrical form which is magnetized at a first, low temperature and subsequently heated to a second, higher temperature which, however, lies below the transition temperature of the HTS; here, no liquid helium is required to generate a strong, static magnetic field. A similar procedure is also described in U.S. Pat. No. 6,545,474 B2.
U.S. 8,061,016 B2 describes a method for fabricating an HTS coil by depositing, shaping and texturing individual coil windings on a curved former. Multi-layer coated cylinders as an alternative method for the fabrication of superconducting coils are described in Maher et al., Supercond. Sci. Tech. 17 (2004), 1440-1445. A method for determining a suitable conductor path in a thin film on a curved former for a superconducting coil has been disclosed in EP 1 604 377 B1.
Kim et al., IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, Vol. 19, No. 3, June 2009, Pages 2273-2276, have proposed charging HTS rings made from solid material in an outer energization magnet and subsequently removing the HTS rings for further use. The HTS rings are cooled with LN2 or helium.
DE 10 2006 012 508 B3 describes a cryostat having a magnetic coil system which is located in a tank with liquid helium. A chamber is installed around the HTS section(s) of the magnetic coil system so that the HTS section(s) are separated from the LTS sections. Liquid helium is contained in the helium tank, whereas the helium remains gaseous in the chamber. The inside of the chamber can be heated with an electrical heater. The LTS sections and the HTS section(s) are electrically connected in series with one another.
A helium-tight bushing through the wall of the chamber is necessary for the serial connection of the LTS sections and the HTS section(s), which is technologically difficult to install. In addition, with this cryostat, a permanent temperature difference exists between chamber and remaining helium tank in operation, which requires a continuously high cooling power and is associated with a high helium consumption.